Capacitance sensor arrangement

Abstract

 A capacitance sensor arrangement is used to determine the distance of an element such as a shearing handpiece from a surface such as the skin of a sheep. The arrangement processes data from one or more sensors (SL, SR, SB) which include a variable capacitor (C_x) dependent on the distance, and provides an output voltage (V_S) consistent with the distance, the output voltage (V_S) being dependent on the voltage drop across a reference capacitor (C_L) in the sensor circuit.

Description

[0001] This invention relates to a capacitance sensor for determining parameter values, and in particular relates to distance measuring in automated sheep shearing.

[0002] AU-A-32064/84 describes a capacitive sensing arrangement for use in sensing the position of a shearing head in an automated sheep shearing system.

[0003] There is described a capacitance sensor per se, an arrangement for locating sensors on a shearing handpiece, means for establishing the distance of the handpiece from the skin of a sheep, by processing information from the sensors.

[0004] The present invention addresses itself to an arrangement for process­ing information from a number of sensors in a capacitive sensing arrangement, which arrangement may be seen as an alternative to the approach taken in AU-A-32064/84.

[0005] The invention provides a capacitance sensing arrangement for indicating the distance of an element from a surface, wherein the voltage drop across a reference capacitor, resulting from the location of sensing means at said distance from said surface, is processed to produce said indication.

[0006] Preferably, the distance is calculated by processing the sensor output voltage V_SL calculated using the equation described hereinafter.

[0007] An embodiment of the invention which may be preferred, will be described in detail hereinafter, with reference to the accompanying drawings, in which:-

Figure 1 is a circuit diagram of a capacitance sensor arrangement; and

Figure 2 is a graph of voltage plotted against distance.

[0008] Referring firstly to Figure 1, the circuit shown incorporates three sensors, a left sensor (SL) a right sensor (SR) and a rear sensor (SB). Each sensor may be of the form shown in Figure 4 of AU-A-32064/84, and may be located respectively on the right, left and rear of a shearing cutter head.

[0009] The capacitance sensor system for measuring distance, in this case from a cutter head to the skin of a sheep, is based on a measurement of voltage drop across a reference capacitor.

[0010] The three sensors (SL, SR, SB) are powered from one local oscillator, which is built on an operational amplifier (IV) which is one element of a quad op-amp (084). With feedback components as shown in the circuit of Figure 1, the oscillator produces a square wave having a frequency of approximately 190 kHz. The possible operating frequency range is limited in practice by the impedances of the capacitors and by the parameters of the operational amplifier.

[0011] The signal from the oscillator is stabilized by temperature compensated Zener diodes (D1,D2), preferably IN827 models. The amplitude is chosen experimentally for the best use of the output voltage swing of the other elements (I, II, III) of the measuring amplifier.

[0012] As an example, we shall only consider the left sensor channel (that for sensor SL). The signal from the oscillator is divided by R9, C_L, C_xp, C_x and R.7 C_L is the fixed reference capacitor, C_xp is a fixed parasitic between the sensor plate and ground, C_x is a variable capacitor dependent on the measured proximity, R7 is a bias resistor and R9 is a resistor added for stability of the amplifier (I).

[0013] The voltage drop across C_L is amplified with a gain established by resistor R5 and resistor R6 together with thermistor Th1. The value of the R-T curve matched thermistor is chosen experimentally to compensate for the thermal drift of the rest of the circuit. The signal is then filtered to remove hum (C2, R8) and converted to D.C. (D3, C3 each a 5082-2811). Resistor R10 is used to control the response time of the converter.

[0014] Neglecting the influence of R9 and R7, the voltage across the capacitor C_L is:

[0015] If the gain of the amplifier (I) together with filter (C2,R8) is K, and the polarizing voltage of D3 is V_p, the sensor output voltage V_SL is given by:

The typical free air voltage, when C_x = 0, is approximately +6V and contact voltage approximately +IIV.

[0016] The sensor output voltage is further processed to calculate the proximity, that is, the distance between the cutter head and the sheep's skin. The on-board oscillator eliminates the need to carry the high frequency signal through parts of a robot control system of which the cutter head may be a part. The oscillator does not have to be individually stabilized; the thermistor compensates for the thermal drift of the output signal. The Op-amp used directly in the measuring circuit, and the filter, produce an improved signal to noise ratio.

Claims

1. A capacitance sensing arrangement for indicating the distance of an element from a surface, wherein the voltage drop across a reference capacitor (C_L), resulting from the location of sensing means at said distance from said surface, is processed to produce said indication.

2. The arrangement of Claim 1, wherein there are three capacitive sensing means (SL,SR,SB), and the voltage drops across respective reference capacitors (C_L) are processed to provide a means distance indication.

3. The arrangement of Claim 1, wherein said distance is calculated from the sensor output voltage (V_S).

4. The arrangement of Claim 3, wherein the output voltage (V_S) is in accordance with the equation

where V_S is the sensor output voltage, K_x is amplifier gain, C_xp is a fixed parasitic between the sensor plate and ground, C_x is a variable capacitor dependent on the measured proximity, C_L is the reference capacitor, and V_p is the polarizing voltage of a diode.

5. A method of sensing the distance between an element and a surface, in which information from a capacitance sensor is processed to produce an indication of distance, the sensor consisting of or including a variable capacitor, the output voltage across a reference capacitor being used to obtain said indication of voltage.

Drawing